Diesters of phthalic acid, particularly dioctyl phthalates (DOP) and the like, are widely used as plasticizers for the processing of many PVC soft plastics, and in the rubber, paint and emulsifier industries, where it can improve the rebound elasticity of products and reduce permanent distortions. In tire treads, in particular, phthalate diester plasticizers can improve snow traction performance. However, due to concerns over the toxicological effects of phthalates, environmental and health groups are now pushing to restrict the use of phthalates, especially in products with direct human contact, such as cosmetics, toys, medical devices and the like.
Although the rubber industry has not yet been directly involved in such environmental concerns, it has been considered prudent by this company to investigate substitutions for phthalate diesters, such as DOP and the like, in rubber compositions. Several substitute compounds have been investigated; however, a one-to-one quantitative substitution of any of these compounds for DOP, that provided the same beneficial rubber properties as DOP, has not been found previously.
Recently, it has been discovered that certain esters of cyclohexanepolycarboxlyic acids are environmentally non-toxic and can be used as plasticizers for polyvinyl chloride to enable products with comparable mechanical properties to be obtained using less polyvinyl chloride. Further, esters and other derivatives of cyclohexanepolycarboxlyic acids have been shown to be useful as components of paints, varnish, inks, adhesives, sealants and the like; as components in lubricating oil or cutting oil, especially in metal working; and as plasticizers in the production of semi-rigid to highly flexible materials, such as medical materials, particularly blood bags, tubing and the like, and cling film for food applications, and the like.
However, the use of such cyclohexanepolycarboxylic acid esters as plasticizers in rubber compounds has not been reported. Nor have the effects of such chemicals on rubber properties been reported. In view of our previously unsuccessful search for non-toxic plasticizers that could be used in quantitative amounts similar to DOP and produce equivalent desirable rubber properties, expectations were not high that cyclohexanepolycarboxylic acid esters would be a suitable substitute for diesters of phthalates, such as DOP and the like, in rubber compounds.